Anapsis (fromAncient Greekἁψίς (hapsís) 'arch, vault';pl. apsides/ˈæpsɪˌdiːz/AP-sih-deez)^[1][2] is the farthest or nearest point in theorbit of aplanetary body about itsprimary body. Theline of apsides (also calledapse line, ormajor axis of the orbit) is the line connecting the twoextreme values.

Apsides pertaining to orbits around different bodies have distinct names to differentiate themselves from other apsides. Apsides pertaining togeocentric orbits, orbits around theEarth, are at the farthest point called theapogee, and at the nearest point theperigee, like with orbits of satellites and the Moon around Earth. Apsides pertaining to orbits around theSun are namedaphelion for the farthest andperihelion for the nearest point in aheliocentric orbit.^[3] Earth's two apsides are the farthest point,aphelion, and the nearest point,perihelion, of its orbit around the host Sun. The termsaphelion andperihelion apply in the same way to the orbits ofJupiter and the otherplanets, thecomets, and theasteroids of theSolar System.

There are two apsides in anyelliptic orbit. The name for each apsis is created from the prefixesap-,apo- (fromἀπ(ό), (ap(o)-) 'away from') for the farthest orperi- (fromπερί (peri-) 'near') for the closest point to theprimary body, with a suffix that describes the primary body. The suffix for Earth is-gee, so the apsides' names areapogee andperigee. For the Sun, the suffix is-helion, so the names areaphelion andperihelion.

According toNewton's laws of motion, all periodic orbits are ellipses. The barycenter of the two bodies may lie well within the bigger body—e.g., the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface.^[4] If, compared to the larger mass, the smaller mass is negligible (e.g., for satellites), then theorbital parameters are independent of the smaller mass.

When used as a suffix—that is,-apsis—the term can refer to the two distances from the primary body to the orbiting body when the latter is located: 1) at theperiapsis point, or 2) at theapoapsis point (compare both graphics, second figure). The line of apsides denotes the distance of the line that joins the nearest and farthest points across an orbit; it also refers simply to the extreme range of an object orbiting a host body (see top figure; see third figure).

Inorbital mechanics, the apsides technically refer to the distance measured between thebarycenter of the 2-body system and the center of mass of the orbiting body. However, in the case of aspacecraft, the terms are commonly used to refer to the orbitalaltitude of the spacecraft above the surface of the central body (assuming a constant, standard reference radius).

The words "pericenter" and "apocenter" are often seen, although periapsis/apoapsis are preferred in technical usage.

• For generic situations where the primary is not specified, the termspericenter andapocenter are used for naming the extreme points of orbits (see table, top figure);periapsis andapoapsis (orapapsis) are equivalent alternatives, but these terms also frequently refer to distances—that is, the smallest and largest distances between the orbiter and its host body (see second figure).
• For a body orbiting theSun, the point of least distance is theperihelion (/ˌpɛrɪˈhiːliən/), and the point of greatest distance is theaphelion (/æpˈhiːliən/);^[5] when discussing orbits around other stars the terms becomeperiastron andapastron.
• When discussing a satellite ofEarth, including theMoon, the point of least distance is theperigee (/ˈpɛrɪdʒiː/), and of greatest distance, theapogee (fromAncient Greek: Γῆ (Gē), "land" or "earth").^[6]
• For objects inlunar orbit, the point of least distance are called thepericynthion (/ˌpɛrɪˈsɪnθiən/) and the greatest distance theapocynthion (/ˌæpəˈsɪnθiən/). The termsperilune andapolune, as well asperiselene andaposelene are also used.^[7] Since the Moon has no natural satellites this only applies to man-made objects.

The wordsperihelion andaphelion were coined byJohannes Kepler^[8] to describe the orbital motions of the planets around the Sun.The words are formed from the prefixesperi- (Greek:περί, near) andapo- (Greek:ἀπό, away from), affixed to the Greek word for the Sun, (ἥλιος, orhēlíos).^[5]

Various related terms are used for othercelestial objects. The suffixes-gee,-helion,-astron and-galacticon are frequently used in the astronomical literature when referring to the Earth, Sun, stars, and theGalactic Center respectively. The suffix-jove is occasionally used for Jupiter, but-saturnium has very rarely been used in the last 50 years for Saturn. The-gee form is also used as a generic closest-approach-to "any planet" term—instead of applying it only to Earth.

During theApollo program, the termspericynthion andapocynthion were used when referring toorbiting the Moon; they reference Cynthia, an alternative name for the Greek Moon goddessArtemis.^[9] More recently, during theArtemis program, the termsperilune andapolune have been used.^[10]

Regarding black holes, the term peribothron was first used in a 1976 paper by J. Frank and M. J. Rees,^[11] who credit W. R. Stoeger for suggesting creating a term using the greek word for pit: "bothron".

The termsperimelasma andapomelasma (from a Greek root) were used by physicist and science-fiction authorGeoffrey A. Landis in a story published in 1998,^[12] thus appearing beforeperinigricon andaponigricon (from Latin) in the scientific literature in 2002.^[13]

The suffixes shown below may be added to prefixesperi- orapo- to form unique names of apsides for the orbiting bodies of the indicated host/(primary) system. However, only for the Earth, Moon and Sun systems are the unique suffixes commonly used.Exoplanet studies commonly use-astron, but typically, for other host systems the generic suffix,-apsis, is used instead.^{[14][failed verification]}

The perihelion (q) and aphelion (Q) are the nearest and farthest points respectively of a body's directorbit around theSun.

Comparingosculating elements at a specificepoch to those at a different epoch will generate differences. The time-of-perihelion-passage as one of six osculating elements is not an exact prediction (other than for a generictwo-body model) of the actual minimum distance to the Sun using thefull dynamical model. Precise predictions of perihelion passage requirenumerical integration.

The two images below show the orbits,orbital nodes, and positions of perihelion (q) and aphelion (Q) for the planets of the Solar System^[18] as seen from above the northern pole ofEarth's ecliptic plane, which iscoplanar withEarth's orbital plane. The planets travel counterclockwise around the Sun and for each planet, the blue part of their orbit travels north of the ecliptic plane, the pink part travels south, and dots mark perihelion (green) and aphelion (orange).

The first image (below-left) features theinner planets, situated outward from the Sun as Mercury, Venus, Earth, and Mars. Thereference Earth-orbit is colored yellow and represents theorbital plane of reference. At the time of vernal equinox, the Earth is at the bottom of the figure. The second image (below-right) shows theouter planets, being Jupiter, Saturn, Uranus, and Neptune.

The orbital nodes are the two end points of the"line of nodes" where a planet's tilted orbit intersects the plane of reference;^[19] here they may be 'seen' as the points where the blue section of an orbit meets the pink.

•  
  The perihelion (green) and aphelion (orange) points of theinner planets of the Solar System
•  
  The perihelion (green) and aphelion (orange) points of theouter planets of the Solar System

The chart shows the extreme range—from the closest approach (perihelion) to farthest point (aphelion)—of several orbitingcelestial bodies of theSolar System: the planets, the known dwarf planets, includingCeres, andHalley's Comet. The length of the horizontal bars correspond to the extreme range of the orbit of the indicated body around the Sun. These extreme distances (between perihelion and aphelion) arethe lines of apsides of the orbits of various objects around a host body.

Distances of selected bodies of theSolar System from the Sun. The left and right edges of each bar correspond to theperihelion andaphelion of the body, respectively, hence long bars denote highorbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.

Currently, the Earth reaches perihelion in early January, approximately 14 days after theDecember solstice. At perihelion, the Earth's center is about0.98329astronomical units (AU) or 147,098,070 km (91,402,500 mi) from the Sun's center. In contrast, the Earth reaches aphelion currently in early July, approximately 14 days after theJune solstice. The aphelion distance between the Earth's and Sun's centers is currently about1.01671 AU or 152,097,700 km (94,509,100 mi).

The dates of perihelion and aphelion change over time due to precession and other orbital factors, which follow cyclical patterns known asMilankovitch cycles. In the short term, such dates can vary up to 2 days from one year to another.^[20] This significant variation is due to the presence of the Moon: while the Earth–Moonbarycenter is moving on a stable orbit around the Sun, the position of the Earth's center which is on average about 4,700 kilometres (2,900 mi) from the barycenter, could be shifted in any direction from it—and this affects the timing of the actual closest approach between the Sun's and the Earth's centers (which in turn defines the timing of perihelion in a given year).^[21]

Because of the increased distance at aphelion, only 93.55% of the radiation from the Sun falls on a given area of Earth's surface as does at perihelion, but this does not account for theseasons, which result instead from thetilt of Earth's axis of 23.4° away from perpendicular to the plane of Earth's orbit.^[22] Indeed, at both perihelion and aphelion it issummer in one hemisphere while it iswinter in the other one. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, regardless of the Earth's distance from the Sun.

In the northern hemisphere, summer occurs at the same time as aphelion, when solar radiation is lowest. Despite this, summers in the northern hemisphere are on average 2.3 °C (4 °F) warmer than in the southern hemisphere, because the northern hemisphere contains larger land masses, which are easier to heat than the seas.^[23]

Perihelion and aphelion do however have an indirect effect on the seasons: because Earth'sorbital speed is minimum at aphelion and maximum at perihelion, the planet takes longer to orbit from June solstice to September equinox than it does from December solstice to March equinox. Therefore, summer in the northern hemisphere lasts slightly longer (93 days) than summer in the southern hemisphere (89 days).^[24]

Astronomers commonly express the timing of perihelion relative to theFirst Point of Aries not in terms of days and hours, but rather as an angle of orbital displacement, the so-calledlongitude of the periapsis (also called longitude of the pericenter). For the orbit of the Earth, this is called thelongitude of perihelion, and in 2000 it was about 282.895°; by 2010, this had advanced by a small fraction of a degree to about 283.067°,^[25] i.e. a mean increase of 62" per year.

For the orbit of the Earth around the Sun, the time of apsis is often expressed in terms of a time relative to seasons, since this determines the contribution of the elliptical orbit to seasonal variations. The variation of the seasons is primarily controlled by the annual cycle of the elevation angle of the Sun, which is a result of the tilt of the axis of the Earth measured from theplane of the ecliptic. The Earth'seccentricity and other orbital elements are not constant, but vary slowly due to the perturbing effects of the planets and other objects in the solar system (Milankovitch cycles).

On a very long time scale, the dates of the perihelion and of the aphelion progress through the seasons, and they make one complete cycle in 22,000 to 26,000 years. There is a corresponding movement of the position of the stars as seen from Earth, called theapsidal precession. (This is closely related to theprecession of the axes.) The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:^[26]

The following table shows the distances of theplanets anddwarf planets from the Sun at their perihelion and aphelion.^[27]

Theseformulae characterize the pericenter and apocenter of an orbit:

Pericenter

Maximum speed,$v_{\text{per}}=\sqrt{\frac{(1+e)\mu }{(1-e)a}}$ , at minimum (pericenter) distance,$r_{\text{per}}=(1-e)a$ .

Apocenter

Minimum speed,$v_{\text{ap}}=\sqrt{\frac{(1-e)\mu }{(1+e)a}}$ , at maximum (apocenter) distance,$r_{\text{ap}}=(1+e)a$ .

While, in accordance withKepler's laws of planetary motion (based on the conservation ofangular momentum) and the conservation of energy, these two quantities are constant for a given orbit:

Specific relative angular momentum

${\displaystyle h=\sqrt{\left(1-e^2\right)\mu a}}$ 

Specific orbital energy

${\displaystyle \epsilon =-\frac{\mu }{2a}}$ 

where:

• $r_{\text{ap}}$  is the distance from the apocenter to the primary focus
• $r_{\text{per}}$  is the distance from the pericenter to the primary focus
• a is thesemi-major axis:

  ${\displaystyle a=\frac{r_{\text{per}}+r_{\text{ap}}}{2}}$ 

• μ is thestandard gravitational parameter
• e is theeccentricity, defined as

  ${\displaystyle e=\frac{r_{\text{ap}}-r_{\text{per}}}{r_{\text{ap}}+r_{\text{per}}}=1-\frac{2}{\frac{r_{\text{ap}}}{r_{\text{per}}}+1}}$ 

Note that for conversion from heights above the surface to distances between an orbit and its primary, the radius of the central body has to be added, and conversely.

Thearithmetic mean of the two limiting distances is the length of the semi-major axisa. Thegeometric mean of the two distances is the length of thesemi-minor axisb.

The geometric mean of the two limiting speeds is

${\displaystyle \sqrt{-2\epsilon }=\sqrt{\frac{\mu }{a}}}$ 

which is the speed of a body in a circular orbit whose radius is${\displaystyle a}$ .

Orbital elements such as thetime of perihelion passage are defined at theepoch chosen using an unperturbedtwo-body solution that does not account for then-body problem. To get an accurate time of perihelion passage you need to use an epoch close to the perihelion passage. For example, using an epoch of 1996,Comet Hale–Bopp shows perihelion on 1 April 1997.^[28] Using an epoch of 2008 shows a less accurate perihelion date of 30 March 1997.^[29]Short-period comets can be even more sensitive to the epoch selected. Using an epoch of 2005 shows101P/Chernykh coming to perihelion on 25 December 2005,^[30] but using an epoch of 2012 produces a less accurate unperturbed perihelion date of 20 January 2006.^[31]

Numerical integration showsdwarf planetEris will come to perihelion around December 2257.^[33] Using an epoch of 2021, which is 236 years early, less accurately shows Eris coming to perihelion in 2260.^[34]

4 Vesta came to perihelion on 26 December 2021,^[35] but using a two-body solution at an epoch of July 2021 less accurately shows Vesta came to perihelion on 25 December 2021.^[36]

Trans-Neptunian objects discovered when 80+ AU from the Sun need dozens of observations over multiple years to well constrain their orbits because they move very slowly against the background stars. Due to statistics of small numbers, trans-Neptunian objects such as2015 TH367 when it had only 8 observations over anobservation arc of 1 year that have not or will not come to perihelion for roughly 100 years can have a1-sigma uncertainty of 77.3 years (28,220 days) in the perihelion date.^[37]

• Distance of closest approach
• Eccentric anomaly
• Flyby (spaceflight)
• Hyperbolic trajectory § Closest approach
• Mean anomaly
• Perifocal coordinate system
• True anomaly

• Apogee – Perigee Photographic Size Comparison, perseus.gr
• Aphelion – Perihelion Photographic Size Comparison, perseus.gr
• Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2020Archived October 13, 2007, at theWayback Machine, usno.navy.mil
• Dates and times of Earth's perihelion and aphelion, 2000–2025Archived October 13, 2007, at theWayback Machine from theUnited States Naval Observatory
• List of asteroids currently closer to the Sun than Mercury (These objects will be close to perihelion)
• JPL SBDBlist of Main-Belt Asteroids (H<8) sorted by perihelion date